defmodule MicrowavepropWeb.GridBounds do
  @moduledoc """
  Bounds clamp + sanity checks for grid-cell HTTP endpoints
  (`/scores/cells`, `/weather/cells`).

  The propagation grid covers HRRR (CONUS) and HRDPS (Canada to
  60°N): roughly lat 25-60, lon -141 to -52. Any caller-provided
  viewport is clamped to a generous superset of that bbox, which is
  what bounds the work: after clamping no request can ask for more
  than the full grid (~92k cells, ~1.4 MB), already materialized once
  per valid_time and served from cache. A global or otherwise absurd
  viewport therefore costs no more than a fully zoomed-out map does.

  `nil` bounds are passed through (caller intends the full grid).
  """

  # Generous superset of HRRR + HRDPS — a few extra degrees on each
  # edge so legitimate clients zoomed slightly past the data extent
  # still get a 200 with the actual covered cells.
  @lat_min 20.0
  @lat_max 65.0
  @lon_min -150.0
  @lon_max -50.0

  @type bounds :: %{optional(String.t()) => float()} | nil

  @doc """
  Clamp `bounds` to the supported grid extent. Returns:

    * `{:ok, nil}` when bounds are nil (caller wants the full grid)
    * `{:ok, clamped_bounds}` otherwise
    * `{:error, :invalid_bounds}` when north < south or east < west

  """
  @spec clamp(bounds()) :: {:ok, bounds()} | {:error, :invalid_bounds}
  def clamp(nil), do: {:ok, nil}

  def clamp(%{"south" => s, "north" => n, "west" => w, "east" => e})
      when is_number(s) and is_number(n) and is_number(w) and is_number(e) do
    south = clamp_value(s, @lat_min, @lat_max)
    north = clamp_value(n, @lat_min, @lat_max)
    west = clamp_value(w, @lon_min, @lon_max)
    east = clamp_value(e, @lon_min, @lon_max)

    if north < south or east < west do
      {:error, :invalid_bounds}
    else
      {:ok, %{"south" => south, "north" => north, "west" => west, "east" => east}}
    end
  end

  def clamp(_), do: {:error, :invalid_bounds}

  defp clamp_value(v, lo, hi), do: v |> max(lo) |> min(hi)
end